Current limiting sand fuse

ABSTRACT

A current limiting fuse having electrical terminals at each end of a tubular housing, a fusible assembly provided within said housing to interconnect the terminals. The fusible assembly including a first fusible element of high current clearing characteristic and a second fusible element of low current clearing characteristic, the first fusible element substantially surrounded by a granular dielectric material, the second fusible element being positioned within an arc extinguishing sleeve formed of a material which is deionizing under the influence of an electrical arc.

United States Patent 1191 Link 1 51 Oct. 8, 1974 [52] US. Cl 337/292,337/161, 337/276 [51] Int. Cl. H0111 85/04 [58] Field of Search 337/162,292, 161, 276,

[56] References Cited UNITED STATES PATENTS 1,965,239 7/1934 Hill337/291 X 2,768,264 10/1956 Jones et a]. 200/144 C 2,809,254 10/1957Edsall 337/162 3,179,773 4/1965 Keeley 337/276 3,287,524 11/1966 Huberet a1. 337/276 X 3,294,936 12/1966 Mikulecky .1 337/297 X FOREIGNPATENTS OR APPLICATIONS 1,513,338 2/1970 Germany ..337/292 614,61512/1948 GreatBritain ..337/292 Primary Examiner-J. D. Miller AssistantExaminer-Fred E. Bell 5 7] ABSTRACT A current limiting fuse havingelectrical terminals at each end of a tubular housing, a fusibleassembly provided within said housing to interconnect the terminals. Thefusible assembly including a first fusible element of high currentclearing characteristic and a second fusible element of low currentclearing characteristic, the first fusible element substantiallysurrounded by a granular dielectricmaterial, the second fusible elementbeing positioned within an arc extinguishing sleeve formed of a materialwhich is deionizing under the influence of an electrical arc.

lit-Claims, 2 Drawing-Figures 1 CURRENT LIMITING SAND FUSE BACKGROUND OFTHE INVENTION Current limiting fuses of the type shown in the MikuleckyUS. Pat. No. 3,294,936 are designed to interrupt currents of overloadproportions as well as currents of short circuit proportions. Thedifficulty encountered in fuses of this type is the interruption ofcurrents of relatively small magnitude of long duration, i.e., a secondor more. The are gap which forms in the small cross sectional areaportions of the fusible element of this type is progressively enlargedby vaporization and the fulgurite is generally continuous. If the arcingis prolonged in interrupting fault currents of relatively smallmagnitude, excessive high are energy is released and intense heating ofthe filler adjacent to the arc region occurs before completion of theinterrupting process. This intense heating and the excessive are energytend to delay cooling of the fulgurite and to allow flow of full orleakage current through the fulgurite often resulting in a restrike andrekindling of the are after initial interruption of the fuse.

In the Mikulecky type current limiting fuse as described in US. Pat. No.3,294,936, lead-tin allow beads are provided in intimate contact witheach silver wire. The lead-tin alloy beads reduce the meltingtemperature of the silver wire due to the formation of an alloy onmeltingof the lead-tin bead on the silver. However, it has been foundthat even though the fusible element fails, the fuse does notnecessarily interrupt current because of the presence of an arc gapwhich is not long enough to obtain arc interruption. If this conditionoccurs for any period of time, the temperature of the fuse can rise totemperatures higher than the destruction temperatures of the surroundingenvironment.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is aside elevational view in section showingthe current limiting fuse of theinvention; and

FIG. 2 is a log log time current curve of fusing elements.

DESCRIPTION OF THE INVENTION The fuse ofthe present invention as seen inFIG. 1 includes a hollow insulating casing or housing ofa predeterminedlength having a conductive ferrule or cap 12 mounted on each end. A fuseassembly 14 is positioned within said casing 10 and interconnects said.caps.

. In accordance with the invention, the fuse assembly 14 includes afirst means 16 of high current clearing characteristic and a secondmeans 18 having a low current clearing characteristic. In regard to theformer, the first means includes an element having a high melting pointtemperature, such as a silver wire which melts at 980 C or copper wirewhich melts at 1,082C. (Silver oxidizes in air and when temperaturerises to a predetermined temperature the silver oxide converts tosilver.) The element 20 depending on the desired current clearingcharacteristic, can be in the form of a perforated or notched strip or aplain wire. The element 20 is electrically connected to the cap 12 andto the plug 21. The element 20 is embedded in a granular refractorymaterial 15 such as sand.

The second means 18 includes a fusible element 20 and an arcextinguishing sleeve 22. The fusible element 20 is formed from amaterial having a low melting point temperature such as tin wire whichmelts at 232C. The element 20 is electrically connected to the cap 12and to a conductive plug 21 at the other end of the sleeve 22.

The fusible element 20 is thermally insulated by means of the arcextinguishing tube or sleeve 22 which is made of a material such assilicone rubber. The inside diameter and length of the tube can beselected to provide a clearing ability for the fuse at voltages andcurrents compatible with the destruction temperatures of the componentsof the surrounding electrical appara tus. The second means 18 can beembedded in the granular refractory material in the sleeve 10. In eithercase the caps 12 should be sealed to the sleeve 10 to enclose said firstmeans 16.

Referring to FIG. 2, the time current characteristic for silver and tinis shown plotted on a logarithmic scale. It will be noted that thecharacteristic for tin has a greater slope than the characteristic forsilver and intersects the silver characteristic. It should be apparentfrom the diagram that the low overload current time characteristicof thetin is being used to limit or interrupt the current in the fuse andthereby prevent the temperature of the fuse from rising to destructiontemperatures. We are also using the high overload or fault current shorttime characteristic of the silver to clear the fuse under fault currentconditions.

It is generally understood that the length of the wire in sanddetermines thev voltage restoration capability of the fuse. In thisregard, it is generally well known that if the wire is too short, therecovery voltage will maintain the element conductive and not allowcomplete interruption of the circuit. The phenomenon which enables thefuse to clearis the ability of the molten silver to form a fulgurite inthe sand when the silver reaches the molten stage. During the formationof this fulgurite heat is absorbed at a very fast rate thus causing aresistance to current flow to build to a point where the voltage acrossthe fuse is not sufficient to maintain a conductive circuit.

Iclaim:

l. A current limiting fuse comprising, a housing, an electrical terminalat each end of said housing, a fusible assembly having one end connectedto one of said terminals and the other end connected to the other ofsaid terminals and within said housing, said fusible assembly formed ofmaterial which is deionizing under the influence of an electrical arcand a granular dielectric material completely filling said housing andsurrounding said assembly.

2. The fuse according to claim 1 wherein said are extinguishing memberis in the form of a sleeve having a length substantially equal to thelength of said second element whereby said second fusible element isinsulated from said granular material so that its long time minimummelting current characteristic is less than said first element.

3. The fuse according to claim 2 wherein said sleeve is formed ofsilicone rubber.

4. A fuse according to claim 1 wherein said second fusible elementcomprises a metallic conductor having a long time minimum meltingcurrent characteristic less than said first fusible element.

5. The fuse according to claim 1 wherein said first fusible element isformed from one of silver and copper wire and said second fusibleelement is formed from tin.

6. The fuse according to claim 1 wherein said arc extinguishing sleevecomprises a silicone rubber tube.

7. A current limiting sand fuse comprising: a hollow casing, aconductive cap at each end of said casing, a

fuse assembly positioned in said casing and having one end connected toone of said caps and the other end connected to the other of said caps,said assembly including first means for providing a high current clearing charcteristic, second means for providing a low current clearingcharacteristic, said first and second means being conducted in series, agranular dielectric material completely filling said casing, and an arcextinguishing sleeve enclosing the full length of said second means forthermally insulating said second means from said granular dielectricmaterial.

8. The fuse according to claim 7 wherein said first means includes afusible element formed from one of silver and copper wire and saidsecond means includes a tin element.

9. A high voltage fuse of the current limiting type, a hollow insulatingcasing, a conductive cap on each end of said casing, a fusible assemblywithin said casing and connected at each end to one of said caps, saidassembly including,

a main fusible element for providing high current 11. The fuse accordingto claim 10 wherein said l metal wire is tin and said insulating sleeveis a silicone rubber tube.

12. The fuse according to claim 9 wherein said main fusible elementincludes a silver wire, said secondary fusible element includes a tinwire connected in series with said silver wire and said thermallyinsulating member comprises a silicone rubber tube.

13. The fuse according to claim 9 wherein said main fusible element issealed within said casing.

PM UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. I 3,840 836 D t d October 8-, 1974 n fl Edwin A. Link It is certified thaterror appears in the above-identified patent and that said Letters.Patent are hereby corrected ae shown below:

Col. 2, line 4, delete "20" and substitute ---'16- C ol. 2, line 6delete Y'ZO" and substitute ---l6--'-; C01. 2, line 8, delete "20" andsubstitute ---l6-- C01; 2,- line 63, after "member" insert enc1osing thefull length of said second fusible element, said arc extinguishingmember Signed and sealed this 4th day of February 1975.

(SEAL) Attest: McCOY M. GIBSON JR. c. MARSHALL DANN v Attesting OfficerI Comnisaioner of Patents H050 UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION Patent No. 3,840,836 Dated October 8, 1974 n fl Eduin A.Li k- It is certified that error appears in the above-identified patentand that said Letters, Patent are hereby corrected as shown below:

Col. 2, line 4, idelete "20" and substitute -'16- C01. 2, line 6, deleteY'ZO" and substitute ---l6-- Col. 2 line 8,' delete "20" and substitute--l6- Col. 2, line 63, after "member" insert --enc1osing the full lengthof said secohd fusible element, said are extinguishing member.

Signed and sealed this 4th day of February 1975.

(SEAL) Attest:

McCOY M. GIBSON 'JR. b c. MARSHA-LL DANN Attesting Officer Commissionerof Patents

1. A current limiting fuse comprising, a housing, an electrical terminalat each end of said housing, a fusible assembly having one end connectedto one of said terminals and the other end connected to the other ofsaid terminals and within said housing, said fusible assembly includinga first fusible element of high current clearing characteristic, asecond fusible element of metallic material of low current clearingcharacteristic, said first and second elements being connected togetherto form a series circuit, and an arc extinguishing member being formedof material which is deionizing under the influence of an electrical arcand a granular dielectric material completely filling said housing andsurrounding said assembly.
 2. The fuse according to claim 1 wherein saidarc extinguishing member is in the form of a sleeve having a lengthsubstantially equal to tHe length of said second element whereby saidsecond fusible element is insulated from said granular material so thatits long time minimum melting current characteristic is less than saidfirst element.
 3. The fuse according to claim 2 wherein said sleeve isformed of silicone rubber.
 4. A fuse according to claim 1 wherein saidsecond fusible element comprises a metallic conductor having a long timeminimum melting current characteristic less than said first fusibleelement.
 5. The fuse according to claim 1 wherein said first fusibleelement is formed from one of silver and copper wire and said secondfusible element is formed from tin.
 6. The fuse according to claim 1wherein said arc extinguishing sleeve comprises a silicone rubber tube.7. A current limiting sand fuse comprising: a hollow casing, aconductive cap at each end of said casing, a fuse assembly positioned insaid casing and having one end connected to one of said caps and theother end connected to the other of said caps, said assembly includingfirst means for providing a high current clearing charcteristic, secondmeans for providing a low current clearing characteristic, said firstand second means being conducted in series, a granular dielectricmaterial completely filling said casing, and an arc extinguishing sleeveenclosing the full length of said second means for thermally insulatingsaid second means from said granular dielectric material.
 8. The fuseaccording to claim 7 wherein said first means includes a fusible elementformed from one of silver and copper wire and said second means includesa tin element.
 9. A high voltage fuse of the current limiting type, ahollow insulating casing, a conductive cap on each end of said casing, afusible assembly within said casing and connected at each end to one ofsaid caps, said assembly including, a main fusible element for providinghigh current clearing characteristics, and a secondary fusible elementconnected in series with said first element for providing low currentclearing characteristics, said secondary element including a low meltingpoint metal wire and a thermally insulating sleeve enclosing the fulllength of said secondary element, and granular inert refractory arcquenching material within said casing embedding said main fusibleelement and said sleeve.
 10. The fuse according to claim 9 wherein saidsecondary fusible element limits the long time temperature rise to alevel of not more than 300* C.
 11. The fuse according to claim 10wherein said metal wire is tin and said insulating sleeve is a siliconerubber tube.
 12. The fuse according to claim 9 wherein said main fusibleelement includes a silver wire, said secondary fusible element includesa tin wire connected in series with said silver wire and said thermallyinsulating member comprises a silicone rubber tube.
 13. The fuseaccording to claim 9 wherein said main fusible element is sealed withinsaid casing.